On Wed, 16 May 2012 15:38:02 -0400, H. S. Teoh <hst...@quickfur.ath.cx>
wrote:
On Wed, May 16, 2012 at 12:48:49PM -0500, Andrei Alexandrescu wrote:
On 5/16/12 12:34 PM, Steven Schveighoffer wrote:
>In other words, ranges aren't enough.
This is copiously clear to me, but the way I like to think about it
is by extending the notion of range (with notions such as e.g.
BufferedRange, LookaheadRange, and such) instead of developing an
abstraction independent from ranges and then working on stitching
that with ranges.
[...]
One direction that _could_ be helpful, perhaps, is to extend the concept
of range to include, let's tentatively call it, a ChunkedRange.
Basically a ChunkedRange implements the usual InputRange operations
(empty, front, popfront) but adds the following new primitives:
- bool hasAtLeast(R)(R range, int n) - true if underlying range has at
least n elements left;
- E[] frontN(R)(R range, int n) - returns a slice containing the front n
elements from the range: this will buffer the next n elements from the
range if they aren't already; repeated calls will just return the
buffer;
- void popN(R)(R range, int n) - discards the first n elements from the
buffer, thus causing the next call to frontN() to fetch more data if
necessary.
On such ranges, what would popFront and front do? I'm assuming since
frontN and popN are referring to how many elements, and since the most
logical definition for elements is bytes, that front gets the next byte,
and popFront discards the next byte. This seems useless to me.
I still don't get the need to "add" this to ranges. The streaming API
works fine on its own.
But there is an omission with your proposed API regardless -- reading data
is a mutating event. It destructively mutates the underlying data stream
so that you cannot get the data again. This means you must double-buffer
data in order to support frontN and popN that are not necessary with a
simple read API.
For example:
auto buf = new ubyte[1000000];
stream.read(buf);
does not need to first buffer the data inside the stream and then copy it
to buf, it can read it from the OS *directly* into buf.
-Steve
